Magnetic properties and structure of palladium/cobalt and palladium/iron multilayers

Magnetic properties and structure of palladium/cobalt and

palladium/iron multilayers

Citation for published version (APA):

Broeder, den, F. J. A., Donkersloot, H. C., Draaisma, H. J. G., & Jonge, de, W. J. M. (1987). Magnetic properties and structure of palladium/cobalt and palladium/iron multilayers. Journal of Applied Physics, 61(8), 4317-4319. https://doi.org/10.1063/1.338459

DOI:

10.1063/1.338459

Document status and date: Published: 01/01/1987

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Magnetic properties and structure of Pd/Co and Pel/Fe multilayers

F.

J.

A. den Broeder and H. C. Donkersloot

Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands H. J. G. Draaisma and W. J. M. de Jonge

Eindhoven University a/Technology, Department of Physics, 5600 ME Eindhoven, The Netherlands Pd/Co and Pd/Fe multilayer films containing ultrathin Co and Fe layers were prepared by vapor deposition on substrates at room temperature. Their modulated structure, even for films containing 2-A-thin Co and Fe layers, was proved by x-ray diffraction and transmission electron microscopy, Below a Co layer thickness of about 8

A,

the Pd/Co multilayers acquire an easy magnetic axis perpendicular to the film, which is mainly caused by magnetic interface anisotropy. This leads for multilayers containing Co monolayers to almost rectangular hysteresis loops, by which these films may be very suitable as a perpendicular magnetic recording medium. Pd/Fe multilayers also have a perpendicular interface anisotropy, but the shape anisotropy dominates. Per unit Co volume the Pd/Co multilayers have a higher saturation magnetization than pure Co, which is attributed to an induced ferromagnetism on

Pd interfacial atoms.

Many years ago, N

eel

I predicted the existence of a

mag-netic surface anisotropy, caused by the reduced symmetry in the surroundings of a surface atom. In principle, such an anisotropy may also be present at the interface between a magnetic and a nonmagnetic metal. In a multilayer structure where there is an abundance of interfaces, it may then affect the magnetic anisotropy of the film as a whole. A large an-isotropy found for compositionally modulated Cu-Ni thin films may have been caused by an easy plane interface anisot-ropy. 2 _{Recently, Carcia, Meinhaldt, and Suna}3 _{reported that} sputtered Pd/Co multilayers with Co layers thinner than 8

A

had an easy magnetic axis normal to the film plane, as a result of a perpendicular interface anisotropy.

The present paper deals with structural and magnetic properties of Pd/Co multilayers prepared by vapor deposi-tion on cold substrates. We found that in these films compo-sition changes are very sharp, resulting in a large interface anisotropy. So far unreported multilayers containing Co monolayers then show almost rectangular hysteresis loops in

perpendicular fields. The obtained results prompted us to investigate also some Pd/Fe multilayers with ultrathin Fe.

The multilayers were prepared in URV bye-beam evap-oration from two sources onto 5i substrates at room tem-perature, with a base layer of 200-A.. Pd. The vapor streams were interrupted alternatingly during predetermined times with mechanically driven shutters, while the deposition rate was kept constant at a fixed value in the range of 0.1-1 A/s as monitored by a quartz resonator.

Table I summarizes constitutional details of the pre-pared Pd/Co films. Chemical analysis of a representative series of multilayers showed that within a few percent the intended amount of Co was present, while that ofPd was 10-20% higher. The multilayer structure was checked by x-ray diffractometry (XRD) using Cuka radiation. Pure Pd films showed a pronounced [111] fiber texture. The multilayers gave near the (111 )Pd peak 2-3 superlattice reflections, which allowed the determination of the bilayer period D. As an example, Fig. 1 shows the XRD profile for a multilayer

TABLE L Constitution and magnetic properties ofPd/Co multilayers; N ~, number ofbilaycrs; t Co = Co layer thickness; t Pd ~~ Pd layer thickness; D = bi· layer period determined by XRD, r, = saturation magnetization per unit Co volume; I R = remanent magnetization; pelf ~ = eoercivity.

t Co t Pd D I, f.l()H~

N (A) cA.)

cA)

(T)

1;/1;

1;//, (Tl

52 12.3 45 66 2.02 0.20 0.09 0.020 34 10.2 45 2.10 0.19 0.07 0.015 56 8.2 45 37 2.02 0.63 0.13 0.045 59 6.2 45 67 1.96 1.8 0.27 0.04 61 4.1 45 58 1.99 7.4 0.45 0.06 100 4 18 26.5 2.70 3.2 0.35 0.09 150 4 9 15.3 2.69 2.1 0.20 0.095 250 4 4.5 8.5 2.36 1.0 0.12 0.08 150 2 18 22.2 2.80 II 0.96 0.16 150 2 13.5 20.0 2.66 9 0.96 0.215 200 2 11.2 17.7 2.8l 13 0.94 0.245 200 2 6.7 9.g 3.05 18 0.96 0.25 200 2 4.5 6.55 2.77 20 0.93 0.245

4317 J. Appl. Phys. 61 (8), 15 April 1987 0021·8979/87/084317-03$02.40 @ 1987 American Institute of PhySics 4317

I

I i

1

20°5; 11

12

f

:i

ro

>. ... til C (JJ ~ .S:

30

35 scattering

FIG.!. XRD profile (Cuka) ofa multilayer containing 4-A CO layers and 18-A Pd layers (N = 1(0). indicated are the substrate (Si,Pd) and themul-tilayer reflections with order numbers L, ddined by 2D sin (Jcc Lt!.

composed of 18-A Pd and 4-A. Co layers. It is to be noted that a Co thickness of

2 A.

is that of a monolayer. As it appears from Table I, D generally exceeds the programmed bilayer period, presumably owing to a higher Pd content

A few samples were also studied by transmission elec-tron microscopy (TEM). Planar sections showed a poly-crystalline fcc structure with a grain size of 200-700

A.

Cross sections of the films revealed a columnar structure with [111] texture, with many [111] microtwins parallel to the multilayer plane. The modulated structure could be made visible in bright field by interference between the pri-mary beam and the L = 1 superlattice reflection, or in dark field by interference between a strong high-order superlat-tice reflection and one of its satellites. As an example, Fig. 2 demonstrates the modulation in a multilayer containing

2-A.

~IG. 2. TEM ;nicrograph ofa cross section ofa multilayer composed of6.5-A Pd and 2-of6.5-A Co layers. The picture is a dark-field image, in which the multilayer fringes at a distance of about 9.5 A were made visible by interfer-ence between the L = 4 and L "'~ 5 superlattice reflections.

4318 J. Appl. Phys., Vol. 61, No.8, 15 April 1987

Co layers. This result provides strong evidence for very steep concentration changes in all the multilayers, possibly occur-ring across one or a few lattice planes.

The magnetic moment of the samples was measured at room temperature with a vibrating sample magnetometer, applying fields up to 1. 7 T, both parallel and perpendicular

to the film plane. The magnetic properties of the Pd/Co

films are collected in Table I.

It appears that the saturation magnetization Is, calcu-lated per unit Co volume, is for all samples significantly higher than for pure Co (I~ = 1.76 T). Since it is well known that in Pd-Co alloys Pd atoms are polarized by neigh-boring Co atoms,4 the present deviation may be explained similarly. Then, assuming that the induced magnetization t:..Is on the Pd atoms is confined to the Pd planes nearest to each Co layer at a distance d;::::: 2.15 A., one should have

Is = I~

+

2AIsd Ileo , (1)

where t Co is the thickness of an individual Co layer. We then find ill, = 0.55

±

0.25 T, which roughly agrees with a quoted value ofO.6.uB per Pd atom as the nearest neighbor of a Co atom in Pd-Co aHoys.4

Figure 3 illustrates the hysteresis curves for a represen-tative series of Pd/Co multilayers. It is seen that with de-creasing t Co the films become easier to magnetize in

perpen-a

_ _ _ '---..l....

0.5 10

-+ 8

0 iTI

FIG. 3. Magnetic hysteresis loops of a series ofPd/Co multilayers, showing the effect of decreasing t ( ' 0 ' They were measured in tields parallel ( ---) and

perpendicular (-) to the film plane and are shown up to B 0 = 1.1 T. The vertical scale is the magnetization per unit Co volume. (al t Co = 12.3 A,

tpd =45A;(b)t_co =8.2At"d =45A;(c)tco =4A,ipd = 18A; (d) teo

= 2 A, tl'd = ! 8 A. To be noted is the almost rectangular shape ofthc per-pendicular loop in case (d).

den Broeder et al. 4318 Downloaded 28 Aug 2011 to 131.155.2.66. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

TABLE II. Constitution and magnetic properties of Pd/Fc multilayers; t Fe = Fe layer thickness; t Pd = Pd layer thickness; D = bilayer period de-termined by XRD; I, = saturation magnetization per unit Fe volume' It"ll ~ = perpendicular field which gives saturation. '

fh tpd D Is {tell;

N

cA)

(A) (A) (T)

en

125 6 18 26.5 2.70 1.1

136 4 18 24.4 2.74 0.85

150 2 18 22.2 2.52 0.46

dicular fields and increasingly more difficult in parallel fields. This leads to a crossover from easy plane to easy axis anisotropy between t Co = 8 and 6

A,

as is demonstrated by

the remanence ratio M

1/

M ~ becoming larger than one (see Table I), This agrees with the results reported for sput-tered Pd/Co multilayers which were prepared down to

t Co

=

4.7

A.

3 In our case the increasing perpendicular

an-isotropy extends even to multilayers with t Co

=

2

A.

For the

latter films the hysteresis curves become almost rectangular with remanences approaching the saturation magnetization [see Fig. 3(d)] They also exhibit high coercivities up to about 0.25 T (2.5 kOe). Inspection of the data in Table I shows that the dramatic increase in perpendicular reman-ence and coercivity when going from t Co = 4

A

to t Co = 2

A

occurs for all multilayers, irrespective of t Pd'

Sputtered Pd/Co multilayers with an easy perpendicu-lar axis have been proposed before as candidate media for perpendicular recording.3

In such an application the high remanences of the vapor-deposited Pd/Co multilavers with

t Co ::::: 2

A

will be beneficial in obtaining a high signai-to-noise

ratio.

In a separate papers it is shown that the uniaxial anisot-ropy constant Kuper unit Co volume, as determined from the paranel and perpendicular magnetization curves of the present Pd/Co multi.layers, satisfies the relation

Ku

=

Kv

+

2Kjtco ' (2)

with Kv = - 0.72 X 1Of:> Jim:; and Ks = O.26x 10-3 _J/m2 ,

Here Kv is a volume contribution per unit Co volume,

inter-preted as the sum of magnetostatic and strain or crystal an-isotropy, while Ks is the Co/Pd interface anisotropy energy. It appears that below t Co = 7.2

A

where K u > 0, the

inter-face term is the major source of the intrinsic anisotropy, The value for Ks is larger than that given (0.16X 10-3 _J/m2

) for

sputtered Pd/Co multilayers3

which is probably due to steeper composition gradients in the vapor-deposited layers. The above results prompted us to investigate a few, simi-larly prepared, Pd/Fe multilayers containing ultrathin Fe layers. Table II lists their constitution and their relevant magnetic properties. XRD demonstrated the modulated structure.

4319 J. Appl. Phys., Vol. 61, No.8, 15 April 1987

Electron diffraction showed the films containing a

2-A

Fe layer to be fcc. TEM results for the films with thicker Fe layers were not yet obtained. It is of great interest to deter-mine whether the Fe atomic planes have an 1lObcc (a-Fe) or

an 111 fcc (y-Fe) symmetry, in view of the controversy on

whether r-Fe is antiferromagnetic,6 or ferromagnetic with a higher magnetic moment than a-Fe. 7.8 Thus the higher l, of

the present Pd/Fe multilayer compared to pure a-FeUs

=

2.15 T) could either be due to polarization ofPd, or

due to r-Fe ferromagnetism. This question is subject of further studies.

The Pd/Fe multilayers appeared to be magnetized more easily in their plane than along their film normal. But Table II also shows that the perpendicular field needed to achieve saturation (p,oH, ) is lower than Is, while this field sharply decreases with lower t Fe. This points to a perpendicular

in-terface anisotropy, which apparently even for the thinnest possible Fe layers does not overcome the high magnetostatic energy associated with a perpendicular magnetization, 5

In conclusion, our structural studies of vapor-deposited I'd/Co and Pd/Fe multilayers indicate that these polycrys-talline films have very sharp interfaces which may extend over only one or a few lattice planes. The saturation magneti-zation of the Pd/Co multilayers per unit volume Co exceeds that of pure Co, presumably by induced ferromagnetism on Pd interfacial atoms, They acquire a perpendicular easy axis below a Co layer thickness of about 8

A,

mainly through the effect of interface anisotropy. For multilayers containing monoatomic Co layers this results in high perpendicular re-manences and coercivities. In Pd/Fe muItilayers a perpen-dicular interface anisotropy is also present, but even for the thinnest Fe layers it does not overcome shape anisotropy.

The authors would like to thank A. Kahle for prepara-tion of the samples, D. Kuiper and H. H. Koek for experi-mental assistance, and U. Enz for stimulating discussions.

'M. L. Ned, J. Phys. Radium 15, 225 (1954).

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3p. J. Carcia, A. D. Meinhaldt, and A. Slilla, Appl. Phys. Lett. 47, ! i8 ( 1985).

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'H. J. G. Draaisma, F. J. A. den Broeder, and W. l. M. de longe, J. Magn. Magn. Mater. (1987) (in press).

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7W. Ktimmerle and U. Gradman, Solid State Commun. 24, 33 (1977). "U. Gradman and H. O. Isbert,J. Magn. Magn. Mater. IS-IS, 1109 (1980).

den Broeder €It a/. 4319